It has been demonstrated that Sendai viral infection produce a rise in the serum amyloid precursor protein (SAA). The time of the peak SAA response is later (9 to 11 days) than the peak of virus production (4 to 5 days). The rise in SAA parallels the development of pneumonia and therefore suggests that inflammation may be required for the production of SAA. An analysis of the course of induction of interferon and that for SAA has been carried out after viral challenge in an attempt to determine whether SAA production is correlated with viremia per se, or with the inflammatory process which is secondary to viremia.